4.18. Debugging the compiler

HACKER TERRITORY. HACKER TERRITORY. (You were warned.)

4.18.1. Dumping out compiler intermediate structures

-ddump-pass

Make a debugging dump after pass
<pass> (may be common enough to need
a short form…). You can get all of these at once
(lots of output) by using
-v5, or most of them with
-v4. Some of the most useful ones
are:

-ddump-parsed:

parser output

-ddump-rn:

renamer output

-ddump-tc:

typechecker output

-ddump-splices:

Dump Template Haskell expressions that we splice in,
and what Haskell code the expression evaluates to.

-ddump-types:

Dump a type signature for each value defined at
the top level of the module. The list is sorted
alphabetically. Using -dppr-debug
dumps a type signature for all the imported and
system-defined things as well; useful for debugging the
compiler.

Show the output of each run of the simplifier. Used when even
-dverbose-core2core doesn't cut it.

-ddump-simpl-iterations:

Show the output of each iteration
of the simplifier (each run of the simplifier has a maximum
number of iterations, normally 4). This outputs even more information
than -ddump-simpl-phases.

-ddump-simpl-stats

Dump statistics about how many of each kind of
transformation too place. If you add
-dppr-debug you get more detailed
information.

-ddump-if-trace

Make the interface loader be *real* chatty about what it is
upto.

-ddump-tc-trace

Make the type checker be *real* chatty about what it is
upto.

-ddump-rn-trace

Make the renamer be *real* chatty about what it is
upto.

-ddump-rn-stats

Print out summary of what kind of information the renamer
had to bring in.

-dverbose-core2core
, -dverbose-stg2stg

Show the output of the intermediate Core-to-Core and
STG-to-STG passes, respectively. (Lots
of output!) So: when we're really desperate:

% ghc -noC -O -ddump-simpl -dverbose-core2core -dcore-lint Foo.hs

-dshow-passes

Print out each pass name as it happens.

-dfaststring-stats

Show statistics for the usage of fast strings by the
compiler.

-dppr-debug

Debugging output is in one of several
“styles.” Take the printing of types, for
example. In the “user” style (the default), the
compiler's internal ideas about types are presented in
Haskell source-level syntax, insofar as possible. In the
“debug” style (which is the default for
debugging output), the types are printed in with explicit
foralls, and variables have their unique-id attached (so you
can check for things that look the same but aren't). This
flag makes debugging output appear in the more verbose debug
style.

-dsuppress-uniques

Suppress the printing of uniques in debugging output. This may make
the printout ambiguous (e.g. unclear where an occurrence of 'x' is bound), but
it makes the output of two compiler runs have many fewer gratuitous differences,
so you can realistically apply diff. Once diff
has shown you where to look, you can try again without -dsuppress-uniques

-dsuppress-coercions

Suppress the printing of coercions in Core dumps to make them
shorter.

-dsuppress-module-prefixes

Suppress the printing of module qualification prefixes in Core dumps to make them easier to read.

-dppr-user-length

In error messages, expressions are printed to a
certain “depth”, with subexpressions beyond the
depth replaced by ellipses. This flag sets the
depth. Its default value is 5.

-dno-debug-output

Suppress any unsolicited debugging output. When GHC
has been built with the DEBUG option it
occasionally emits debug output of interest to developers.
The extra output can confuse the testing framework and
cause bogus test failures, so this flag is provided to
turn it off.

4.18.3. How to read Core syntax (from some -ddump
flags)

Let's do this by commenting an example. It's from doing
-ddump-ds on this code:

skip2 m = m : skip2 (m+2)

Before we jump in, a word about names of things. Within GHC,
variables, type constructors, etc., are identified by their
“Uniques.” These are of the form `letter' plus
`number' (both loosely interpreted). The `letter' gives some idea
of where the Unique came from; e.g., _
means “built-in type variable”; t
means “from the typechecker”; s
means “from the simplifier”; and so on. The `number'
is printed fairly compactly in a `base-62' format, which everyone
hates except me (WDP).

Remember, everything has a “Unique” and it is
usually printed out when debugging, in some form or another. So
here we go…

(“It's just a simple functional language” is an
unregisterised trademark of Peyton Jones Enterprises, plc.)

4.18.4. Unregisterised compilation

The term "unregisterised" really means "compile via vanilla
C", disabling some of the platform-specific tricks that GHC
normally uses to make programs go faster. When compiling
unregisterised, GHC simply generates a C file which is compiled
via gcc.

Unregisterised compilation can be useful when porting GHC to
a new machine, since it reduces the prerequisite tools to
gcc, as, and
ld and nothing more, and furthermore the amount
of platform-specific code that needs to be written in order to get
unregisterised compilation going is usually fairly small.

Unregisterised compilation cannot be selected at
compile-time; you have to build GHC with the appropriate options
set. Consult the GHC Building Guide for details.